1. Field of the Invention
This invention relates to a method for the production of maleic anhydride which comprises a step for the azeotropic dehydration of a crude maleic acid containing aqueous solution, and more particularly to a method for the production of maleic anhydride, characterized by azeotropic dehydration by using an organic solvent having the maximum concentration in water in the range of 0.1-5% by weight at a temperature of 20° C.
2. Description of Related Arts
The maleic anhydride is a general-purpose compound which is possessed of a double bond of large reactivity and two carboxyl groups in the form of an anhydride and, therefore, is capable of chemically performing various reactions and is extensively used in various fields covering plasticizers and agricultural pesticides as well as pharmaceutical preparations, polyester resins, and alkyd resins. This maleic anhydride is produced by a method which comprises oxidizing such an aliphatic hydrocarbon as n-butane which has not less than four carbon atoms or benzene in a reactor for catalytic gas phase oxidation, recovering maleic acid from the produced gas containing maleic anhydride and maleic acid, and refining the recovered maleic anhydride. A water which obtained by washing the waste gas discharged during the production of phthalic anhydride by the reaction of catalytic gas phase oxidation of naphthalene or o-xylene contains maleic acid in an appreciable quantity. The maleic anhydride, therefore, is produced by a method which comprises recovering the water and changing maleic acid to maleic anhydride.
Generally, when maleic anhydride is produced via the reaction of catalytic gas phase oxidation, the method which directly subjects the maleic anhydride obtained by the reaction of catalytic gas phase oxidation in its unmodified form to purification and which absorbs the obtained maleic anhydride once in an aqueous solution and purifies the aqueous solution containing the crude maleic acid, subsequently converts maleic acid into maleic anhydride are available, for example.
JP-B-41-3172, for example, discloses as a means to produce maleic anhydride by subjecting a mixed gas of benzene and air to catalytic gas phase oxidation, a method for continuous production of maleic anhydride which comprises exposing a reaction gas containing maleic anhydride to water thereby obtaining a crude maleic acid containing aqueous solution, dissolving the aqueous solution in fused maleic anhydride, and continuously adding the resultant mixed solution into a mixed solution of maleic anhydride and aromatic hydrocarbon at a temperature in the range of 130-160° C. thereby effecting liquid-phase dehydration of maleic acid. The aromatic hydrocarbons used for the purpose of dehydrating maleic anhydride include xylene, cymen, ethyl benzene, diethyl benzene, and dichlorobenzene, for example. The quantity of the aromatic hydrocarbon used for this purpose varies with the kind of the azeotropic solvent to be used. Generally, it is 2-5 times the quantity of the water which is formed in consequence of the dehydration of maleic acid.
JP-A-50-50316 discloses a method for obtaining maleic anhydride by absorbing in water a maleic acid-containing gas resulting from the catalytic oxidation of an aliphatic or aromatic hydrocarbon thereby forming a crude maleic acid-containing aqueous solution, treating the aqueous solution at a temperature in the range of 100-150° C. under a pressure in the range of 400-760 mmHg thereby deriving fused maleic acid, and subjecting the fused maleic acid to distillation for dehydration at a temperature in the range of 115-165° C. under a pressure in the range of 40-200 mmHg. This method is characterized by converting part of maleic acid into maleic anhydride, treating the impurities consequently entrained with water thereby forming a solid phase composed of fumaric acid and other insoluble impurities and an aqueous liquid phase containing maleic acid, expelling the solid phase from the system by filtration, and recirculating the aqueous filtrate of maleic acid for purification. This method is directed to precluding the method for continuous production of maleic anhydride from inducing faults such as gradual deposition of impurities on the inner walls of devices, blockage of such devices due to the deposition, and degradation of the conduction of heat.
Further, JP-A-63-313782 discloses a method for the production of maleic anhydride which is directed, while the washings of waste gas containing organic substances comprising maleic acid as a main component at a concentration in the range of 20-40% by weight is azeotropically distilled with o-xylene in a dehydrating column to convert the maleic acid into maleic anhydride and obtain maleic anhydride, to removing with a vacuum evaporator the pitch entrained by the maleic anhydride withdrawn from the dehydrating column through the lower part thereof.
When maleic anhydride is obtained by the reaction of catalytic gas phase oxidation, however, the apparatus for continuous production frequently suffers the interior thereof to incur blockage with such impurities as benzoquinone which is by-produced in the reaction and with fumaric acid which is by-produced in the process for purification of maleic anhydride, for example. While JP-A-50-50316mentioned above teaches a method for precluding such a method for continuous production of maleic anhydride from inducing faults such as gradual deposition of impurities on the inner walls of devices, blockage of such devices due to the deposition, and degradation of the conduction of heat, the method has not been capable of preventing the blockage fully satisfactorily. The method disclosed by JP-A-41-3172 falls short of satisfaction.
As a means to repress the occurrence of this blockage chemically unlike the methods described above, JP-B-03-76311 discloses a method for the production of maleic anhydride which is directed, during the production of maleic anhydride as from a crude maleic acid-containing aqueous solution obtained by absorbing in water the reactive gas resulting from the reaction of catalytic gas phase oxidation of benzene and a C4 hydrocarbon fraction, toward concentrating and dehydrating the crudemaleic acid-containing aqueous solution by the addition of an aqueous hydrogen peroxide solution. According to this patent publication, the crude maleic acid-containing aqueous solution contains various kinds of impurities and these impurities are suffered to occur in the form of a mixture of intermediate products and by-products without reference to the kind of hydrocarbon to be used as a raw material and are not perfectly prevented easily even by such means as modification of a reaction catalyst. It offers an explanation purporting that the cause for the persistence of these impurities is ascribable to the promotion of resinification or gelation by the cooperation of a phenol and an aldehyde or a quinone and an aldehyde, for example, and that the apparatus is consequently blocked. It is held that the formation of a resinous or gel-like substance can be prevented by adding hydrogen peroxide to the crude maleic acid-containing aqueous solution. This prevention, however, entails a new problem that the apparatus for purification is corroded by the formic acid which is inevitably by-produced. Further, the hydrogen peroxide is known as a polymerization initiator for maleic anhydride. The azeotropic distillation for dehydration which has a particularly high working temperature, therefore, newly forms a maleic acid polymer which is destined to serve as a blocking substance.
This invention, in view of the problems mentioned above, contemplates providing such a method for the production of maleic anhydride as allows a protracted continuous operation for the process of purification which is capable of preventing the generation of a blocking substance without inducing relevant devices to corrode, preventing the deposition of a blocking substance in a column for azeotropic dehydration and distillation which, because of a complicated internal structure, renders the work of washing the interior of the column irksome and difficult, and removing the impurities by-produced in the reaction.